What's the deal with Xbox Achievements

You know the ones that are one the hard drive. Can you buy stuff with it or what?

No you can't buy anything with them. Achievements give bragging rights for hard to get achievements, but other then that its just a number next to your gamertag. To buy things on the marketplace, you need a microsoft points which you can buy at walmart or wherever you shop, also you can buy them with your credit card in the Xbox Live Market Place.